Create a directory

First, you have to create a directory to hold all stuff specific for your target device.

mkdir vendor/xxx/yyy

or

mkdir device/xxx/yyy

where xxx is usually the vendor's name and yyy is the product name. Historically, we used the vendor/ directory. However, Google suggested to use device/ since Froyo. We encourage to use the new convention device/ as AOSP does.

Create makefiles

At least you have to create four makefiles in the new directory:

AndroidProducts.mk

This makefile should only contain PRODUCT_MAKEFILES which is a list of product makefiles to expose to the build system. So if your product name is yyy, it is

You have to define PRODUCT_PACKAGES, PRODUCT_NAME, PRODUCT_DEVICE in it, and call inherit-product to inherit from a product makefile.
I suggest to to inherit generic_x86.mk to get all common x86 rules.

$(call inherit-product,$(SRC_TARGET_DIR)/product/generic_x86.mk)

The board config makefile: BoardConfig.mk

You can override some variables before including $(GENERIC_X86_CONFIG_MK)

- TARGET_HAS_THIRD_PARTY_APPS
set to true if you hope to compile third party apps like ConnectBot, FileManager introduced by Android-x86.
Default: false

- BOARD_USES_ALSA_AUDIO
Set to false if you don't want ALSA audio.
Default: true.

- BOARD_HAVE_BLUETOOTH
If you don't need bluetooth, set it to false.
Default: true.

- BOARD_USES_TSLIB
Set to true if you need tslib to calibrate the touchscreen.
Default: false

You can override some variables before including
$(GENERIC_X86_ANDROID_MK)

- LOCAL_FIRMWARES
Specify the firmwares to be included in the target. The firmwares are supposed to be already in build/target/board/generic_x86/firmware/.

You may also specify a different directory in LOCAL_FIRMWARES_DIR.

- TARGET_INITRD_SCRIPTS
Specify a hook script to be added in the initrd.img. Usually the script should contain a detect_hardware function that will be called by the init script.
Read eeepc_info as an implementation example.

- TARGET_PREBUILT_APPS
If you want to include some prebuilt apps (apk files), set them to this variable.

- TARGET_KERNEL_CONFIG
Specify a kernel config file for this target. If not set, the default android-x86_defconfig will be used.

Besides the four makefiles, you may also add the file init.yyy.rc to do some initializations for your product
The file will be included automatically if exists,and called by Android init on booting(after executing init.rc).